Membrane proteins

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Membrane proteins are proteins associated to their lipid bilayer in some way, to provide that membrane with its characteristic functional properties. The amount and types of proteins in a membrane are highly variable, for example in a typical plasma membrane about 50% mass is membrane proteins, whereas in plasma membranes involved in ATP production there is around 75% mass.

Membrane proteins can be organised into two main categories- integral (intrinsic) proteins and peripheral (extrinsic). They are organised based on their membrane-proteins interactions. Integral membrane proteins have portions embedded in the phospholipid bilayer. An example of intrinsic proteins are transmembrane and lipid-linked proteins. Peripheral membrane proteins are bound to the membrane either indirectly via interactions with integral membrane proteins or directly via interactions with lipid polar head groups. Protein attached proteins are examples of extrinsic proteins[1].

There are many ways that the membrane proteins can be associated to the membrane. Transmembrane proteins are directly placed in the lipid bilayer and contain a hydrophobic region within the phospholipids and hydrophillic regions in the cytosol and outside the cell. However, some membrane proteins are non covalently bonded to other proteins embedded in the lipid bilayer[2]. These proteins are known as peripheral membrane proteins, these proteins do not extend into the hydrophobic interior of the bilayer. This type of protein can be released from the bilayer by simple procedures, which affect the protein-protein interactions. Whereas transmembrane proteins also known as integral membrane proteins, cannot be released by simple procedures[3].

Membrane proteins are proteins associated to their lipid bilayer in some way, to provide that membrane with its characteristic functional properties. The amount and types of proteins in a membrane are highly variable, for example in a typical plasma membrane about 50% mass is membrane proteins, whereas in plasma membranes involved in ATP production there is around 75% mass.

Membrane proteins can be organised into two main categories- integral (intrinsic) proteins and peripheral (extrinsic). They are organised based on their membrane-proteins interactions. Integral membrane proteins have portions embedded in the phospholipid bilayer. An example of intrinsic proteins are transmembrane and lipid-linked proteins. Peripheral membrane proteins are bound to the membrane either indirectly via interactions with integral membrane proteins or directly via interactions with lipid polar head groups. Protein attached proteins are examples of extrinsic proteins[4].

The different ways the protein can associate with the cell surface membrane can be linked to its function. For example, a receptor protein could be a transmembrane protein as it would receive a signal from the extracellular space, and then generate a response in the form of intracellular signals. Furthermore, a transporter protein would also be a transmembrane protein, in order to provide a pathway for a molecule to cross the hydrophobic permeability barrier caused by the lipid bilayer. On the other hand, an example of a peripheral protein could be an enzyme, such as a kinase, which would catalyse a reaction only on the intracellular side of the cell surface membrane.

References

  1. Molecular Cell Biology. 4th Edition. Lodish H, Berk A, Zipursky SL, et al. W. H. Freeman. New York. 2000. Section 3.4
  2. Alberts et al (2008). Molecular Biology of the Cell. 5th ed. New York: Garland Science. 629-630.
  3. Alberts et al (2008). Molecular Biology of the Cell. 5th ed. New York: Garland Science. 630.
  4. Molecular Cell Biology. 4th Edition. Lodish H, Berk A, Zipursky SL, et al. W. H. Freeman. New York. 2000. Section 3.4
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